Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and one or more rotor blades. The rotor blades capture kinetic energy from wind using known foil principles and transmit the kinetic energy through rotational energy to turn a shaft coupling the rotor blades to a gearbox, or if a gearbox is not used, directly to the generator. The generator then converts the mechanical energy to electrical energy that may be deployed to a utility grid.
Wind turbine blades are generally designed for an optimal wind speed and, thus, are less efficient at other wind speeds. Prior attempts to increase the effective range of wind speeds for the turbine blades have involved active systems that modify or change the aerodynamic profile of the blade by moving or adjusting appendages, flaps, or other control surfaces attached to the blades in an active feedback control loop. Electro-mechanical systems are typically incorporated within the blade for moving the control surfaces. Reference is made for example to U.S. Pat. No. 7,922,450, which describes a blade with a trailing edge section that is moved with an internal piezo-electric actuator in response to aerodynamic loads on the blade.
These systems have also been introduced for load control purposes, wherein the load on the blades is reduced in high wind conditions by changing the aerodynamic profile of the blade via the active control surfaces. Efforts have been made to increase the energy output of wind turbines by increasing the length and surface area of the rotor blades. However, the magnitude of deflection forces and loading of a rotor blade is generally a function of blade length, along with wind speed, turbine operating states, blade stiffness, and other variables. This increased loading not only produces fatigue on the rotor blades and other wind turbine components but may also increase the risk of a sudden catastrophic failure of the rotor blades, for example when excess loading causes deflection of a blade resulting in a tower strike.
Load control is thus a crucial consideration in operation of modern wind turbines. Besides active pitch control systems, it is also known to vary the aerodynamic characteristics of the individual rotor blades as a means of load control, for example with controllable vortex elements, flaps, tabs, and the like configured on the blade surfaces. For example, U.S. Pat. No. 6,972,498 describes various wind turbine blade configurations wherein a retractable extension is provided on a base blade segment to reduce the effective length of the blade in high load conditions. In a particular embodiment, the blade extension is hinged to the base blade segment and jackknifes between a fully extended position and a fully retracted position wherein the blade extension folds into the base blade segment.
Accordingly, the industry would benefit from an improved wind turbine blade design that has an increased effective wind speed range yet avoids the expense and relatively complicated components associated with active enhancement systems.